catalyzed by an enxyme called "Dehydrogenase" which removes two electrons and two hydrogens from some substrate molecule and then trasfers both electrons and either one or both hydrogens to a coenzyme

Photosynthesis

(Coenzymes)

NADP+ (oxidized form) --> NADPH (reduced form)

Cellular Respiration

NAD+ (oxidized form) --> NADH (reduced form)

FAD (oxideized form) --> FADH2 (reduced form)

Photosynthesis Pigments

Chlorophyll-a

Chlorophyll-b

Xanthrophyll

Carotens

The photosynthetic pigments are located embedded in the phospholipic bilayer of the thylakoid membranes and are organized into clusters called photosystems.

Photosystem

a complex of light-gathering "antemae" molecules and a "reaction center" which is a special chlorophyll-a molecule located in close proximit to a "primary electron acceptor" molecule.

There are two photosystems (1 and 2).

Photosystem-1 (PS-1)

Has a reaction center chlorophyll-a molecule called P700.

Photosystem-2 (PS-2)

Has a reaction center chlorophyll-a molecule called P680.

Photooxidation of Chlorophyll

When the energy from a photon of light is absorbed by the reaction center chlorophyll-a molcule, it makes it possible for an electron to go from its normal "ground state" to a higher energy "excited state" orbital.

Before the electron can give off the energy and return it to its more stable lower-energy ground state, the elctron is stolen by the primary electron acceptor an entered into an electron trasport chain.

Electron Transport Chain (ETC)

a series of membrane protens, each one being more electronegative than the previous one.

as electrons are pulled exergonically down the ETC the energy is used to actively pump hydrogen ions (H+) from the stroma into the Thylakoid space to establish a proton gradient.

The potention energy of the proton gradient is converted to kinetic energy as the H+ spill back into the stroma through membrane protein cores called ATP-Synthase.

ATP Synthase

uses the kinetic energy from the flow of H+ to phosphorylate ADP and from ATP.

Cyclic e- flow

Involves only photosystem-1 (PS1)and P700

Photooxidation of PS1 (P700) electrons into an ETC that terminates with P700

The exergonic form of e- from P700 back to P700 is used to generate ATP

NADPH is formed

No O2 is formed

Noncyclic e- flow

Involves both PS-1 (P700) and PS-2 (P680)

Photooxidation of PS-1 (P700) sends e- into an ETC that terminates with NADP+ reducing it to NADPH

Photooxidation of PS-2 (P680) sends e- into an ETC that terminates with P700 and generates ATP in the process

Through a process known as "Photolysis" , a photon of light is used to split a water molecule into 2H+, 2e-, and oxygen.

The 2H+ are released into the stroma, the 2e- are given to P680 to replace the electrons lost to P700, and the oxygen atom combines with another oxygen atom to form O2 which is realsed as waste

Dark Reactions Stroma

The dark reactions begin when Ribulose Biphosphate Cazboxylase (or "rubP carboxylase" or "Rubisco") picksup a molecule of CO2 from the leaf air spaces and fixes (attatches) it to a 5-carbon compound already in the calvin cycle called ribulose biphosphate (or "RuBP")

The resukting 6-carbon molecule is unstable and immediately spilts into two stable 3-carbon molecules called 3-Phosphoglycerate

Atp is used to phosphorylate each 3-Phosphoglycerate to Glyceraldehyde-3 Phosphate (or "G3P") which is the actual product of the calvin cycle

At this point, one molecule of G3P leaves the calvin cycle as a product and additional ATP is udes to convert any remaining G3P molecules back into RuBP to complete the cycle

Photorespiration

an ineffective metabolic pathway that consumes O2 and ATP, evolves CO2, does not produce any carbohydrate and robs the calvin cycle

Photorespiration

It occurs because Rubisco's active site will accept either O2 or CO2 with equal affinity.

When Rubisco picks up O2 and attatches it to RuBp, the resulting unstable 5-carbon compound splits into a 3-carbon molecule called 3-phosphoglycerate and 2-carbon compound called glycolate.

3-phosphoglycerate remains in the calvin cycle but glycolate exits the cycle and goes to a peroxisome and then to a mitochondrion where it completely oxidized to two molecules of CO2.

C4 and CAM photosynthesis both evolved as a way to by-pass photorespiration

Photorespiration typically occurs under dry conditions when plants are forced to close their leaf stromata (openings) prematurely.

When this occurs, the O2 concentration in the leaf air spaces increases while the CO2 concentration decreases, favoring rubisco picking up O2

Occurs only in the mesophyll cells and uses pepcarbolase instead of Rubisco.

Pep-Carboxylase (phosphoenyl carboxylase) picks up a molecule of CO2 it attatches it to a 3-carbon molecule called phosphoenyl pyruvate (pep) which forms a stable 4-carbon intermediate called oxaloacatate.

Oxaloacetate is then converted to another 4-carbon oraganic acid called malate which is shuttled from a mesophyll cell and into a bundle sheath cell via a plasmodesma.

Once inside the bundle sheath cell, t he malate is broken down into a 3-carbon molecule called pyruvate and a 1-carbon molecule CO2

The CO2 is picked up by Rubisco and fixed a second time into carbohydrates via the calvin cycle

The pyruvate molecule is shuttled back into the bundle sheath cell where ATP is used to phosphorylate it into pep to complete the cycle

CAM Photosyntheiss "Crassulacean Acid Metabolism"

CAM plants use a "temporal" separtion of primary and secondary carbon fixation events

Primary Carbon Fixation

fixes CO2 unto organic acids like malate, occurs at night when the stomata are open

The malate is stored in the central vacuole.

Secondary Carbon Fixation

occurs in the datytime via the calvin cycle when the stored malate is broken down into pyruvate and CO2 while the stomata are closed.

involves using the kinetic energy from the exergonic flow of electrons down the ETC to pump H+ against their concentration gradient and then converting the potential energy of the H+ to phosphorylate ADP to form ATP.

4-Phases of Cellular (Aerobic) Respiartion

Glycolysis

Oxidation of Pyruvate to Acetyl Coenzume-A (Acetyl CoA)

Kreb's Cycle

OP/ETC

Glycolysis

Occurs in the cytoplasm (cytosol) whether O2 is present or absent.

One 6-carbon glucose molecule is oxidized and split into 3-carbon pyruvate molecules

2 ATP are consumed

4 ATP are produced by SLP (2 net gain/yeild)

2 NAD+ are reduced to2 NADH (each one is worth 2 ATP by OP/ETC)

No FADH2 are produced

No CO2 are formed/evolved released

Fate of Pyruvate

Depends on the presence/ absence of O2

If O2 is absent (an aerobic) pyruvate remains in the cytosol and is reduced to either ethanol/locate through a metabolic process known as fermentation.

If O2 is present (Aerobic) pyruvate enters the mitochondrial matrix and is oxidized to Acetyl CoA. (2-C)

Oxidation of Pyruvate to Acetyl CoA

Occurs in the mitochondrial matrix only f O2 is present

Each 3-carbon pyruvate is oxidized to a two carbon molecule called acetyl CoA

One molecule of CO2 is formed/evolved

One molecule of NAD+ is reduced to NADH and is worth 3 ATP through OP/ETC

No ATP is produced by SLP

No FADH2 is formed

Kreb's Cycle

Occurs in the mitochondrial matrix if O2 is present.

For every 2 carbon acetly CoA that enters the cycle:

-2 CO2 are formed/evolved

-1 ATP is formed by SLP

-1 FADH2 is formed (worth 2 ATP throguh OP/ETC)

-3 NAPH are formed (each one is worth 3 ATP each through OP/ETC)

OP/ETC

Located embedded in the inner mitochondiral membrane (cristae)

Occurs only if O2 is present (aerobic)

electrons are entered into the chain, from the matrix side, by both FADH2 and NADH

The exergonic flow of e- down the ETC is used to pump H+ gradient in the intermembrane space is converted to kinetic energy as the H+ spill back into the matrix through membrane protein channels called ATP synthase

The kinetic energy from the exergonic flow of H+ through ATP synthase is used to phosphorylate ADP to for ATP

Since NADH enters its electrons into the beginning of the chain, enough H+ are pumped into the intermembrane space to perform 3 phosphorylation reactions (each NADH is worth 3 ATP)

Since FADH2 enters its electrons further down the chain, its electrons into the intermembrane space to perfom 2 phosphorylation reactions (each FADH2 is worth 2 ATP)

Exception:

The NADH fromed during glycolysis are each worth only 2 ATP (because the inner mitochondrial membrane is impermeable to NADH, so NADH sends its electrons across the inner membrane and itno the matrix where they are picked up by FAD, reducing it to FADH2, which enters the electrons into the ETC).

Fermentation

If O2 is absent, glycolysis sitll occurs and splits glucose into two molecules of pyruvate

Instead of entering the mitochondrion and being oxidized to acytyl CoA, pyruvate remains in the cytosol and is reduced to either ehtanol or lactate as a way of regenerating NAD+ from NADH.

The process is known as fermentation and the NADH formed during glycolysis serves as the reducing agent while pyruvate serves as the oxidizing agent

No ATP are consumed/produced during the process and no NADH or FADH2 are formed